In general, mechanical shield excavation of a curved tunnel is conducted by advancing a shield along the curve by uneven operation of shield jacks acting on different points on the shield, while conducting over-break by means of an over cutter or a copy cutter. With such a known method, however, it is impossible to reduce the radius R of curvature to a level below a certain value, e.g., 100 meters. In order to execute curved tunnel excavation, therefore, an articulated shield tunneling machine has been employed in which the shield is composed of two or three sections which are articulated one to another so as to allow flexing of the shield at articulates between, for example, the front and the middle shield sections and between the middle and rear shield sections.
The articulated shield composed of two shield sections, i.e., a front section and a rear section, are sorted into two types: namely, a first type in which shield jacks are held on the front shield section and a second type in which the shield jacks are held by the rear shield section. The first type of the articulated shield is exemplarily shown in FIG. 1. As will be seen from this Figure, shield jacks 27 are supported by the front shield section 21a. A too large flexing angle .alpha. of shield section 21b undesirably causes the radially inner shield jack 27 to interfere with the front end of the rear shield section 21b. In addition, the point 27b of action of the force exerted by the radially inner shield jack on the segment 29 in the rear shield section 21b is offset towards the center of the rear shield section 21b with the result that a reactional force is produced to deform the segment 29. On the other hand, the distance between the radially outer shield jack 27 and the associated segment 29 is increased so that the end of the shield jack 27 may fail to reach the segment 29 even when the shield jack is fully extended.
The second type of shield has the shield jacks 27 held on the rear shield section 21b, as shown in FIG. 2. This type of shield also encounters a problem in that a too large flexing angle .alpha. of the front shield section 21a with respect to the rear shield section 21b undesirably causes one of the shield jacks 27 to interfere with the front end of the front shield section 21a. Anyway, with known articulated shields composed of front and rear shield sections, it has been impossible to excavate a tunnel along a curve having a small radius of curvature, e.g., 30 meters or less.
In order to avoid interference between the front end of the shield jack 27 and the front shield section 21a, it would be advisable to mount the shield jack 27 at an offset towards the center of the rear shield section, as proposed in Japanese Utility Model Unexamined Publication No. 59-167891. In such a case, however, the offset .epsilon. of the point of action of force on the segment 29 from the axis of the shield jack 27 becomes large so that the shield jack is required to have a construction which is strong and, hence, expensive.
On the other hand, an articulated shield composed of three shield sections is capable of performing excavation along an acute curve which has a small radius of curvature, e.g., 30 meters or smaller, at a cost that the overall length of the shield becomes large as compared with articulated shields having two sections, and pairs of shield jacks are required on both ends of the central shield segment, resulting in a complicated and expensive construction as compared with the articulated shield having two sections.